Exposure timing mechanism



Patented Mar. 8, 1949 EXPOSURE TIMING MECHANISM Walter L. Linde,Brooklyn, N. Y., assignor to Pavelle Color Incorporated, New York, N.Y., a

corporation of Delaware Application February 13, 1948, Serial No. 8,268

2 Claims.

The present invention relates to timing mecha nisms of projectionprinters, used to control we exposure period in the photographicreproduction of a transparent picture by passing printing light throughthe picture to light sensitive paper or film material on which thepicture is reproduced. In the operation of a projection printer of theabove-mentioned type, the duration of the exposure period required foroptimum results in printing a picture with light of standard intensity,depends primarily on three factors; namely, on the character of thelight sensitive material and particularly on the sensitivity or speed ofthe light sensitive emulsion coating of said material, on the averagedensity of the negative or other transparency bearing the picturereproduced, and on differences between the densities of differentportions of the picture. Thus, for example, if some portion of thepicture,,such as a child's face, is of special interest, the exposureperiod should be longer when that portion of the picture is relativelydense than when it is relatively transparent, even though the averagedensity of the picture is the same in each case.

A primary object of the present invention is to provide improvements inan automatic timing mechanism for projection printers of the typedisclosed in an article by J. Robins and L. E. Varden, printed in theJune, 1946, issue of the periodical "Electronics. That timing mechanismincludes means for projecting light from the source of printing lightthrough a transparent picture being reproduced to a photocell to therebyproduce a photocell current of a magnitude which depends on the averagedensity of the transparent picture and which is used to discharge atiming condenser previously given a regulated charge.

Since the photocell current is a function oi the intensity of the lightreceived by the photocell,

the time required to discharge a condenser charge oLgiven magnitudevaries wtih variations in the average density of the picture beingreproduced.

In timing mechanisms of the above-mentioned type heretofore used, thecorrective changes in the exposure period to compensate for variationsin the sensitivity or speed of the photographic ma- 2 prises a pluralityof resistor elements permanently connected in series. One of saidvoltage dividers is connected across a source of constant voltage, andthe other voltage divider is connected in shunt to a variable portion ofthe other voltage divider. In prior timing mechanisms of theabove-mentioned type, the compensating variations made in the chargesput on the timing condenser, were effected by adjustments of a singlevoltage divider connected across a source of constant voltage, and theadjustments made included the operative connection in and removal fromthe voltage divider of a plurality of resistor sections, with acorresponding change in the voltage drop across each resistor sectionoperatively retained than any suggested in .the above-mentioned.

- "Electronics" article, and too high to permit those terial emulsionand for differences between theaverage density, of the transparentpicture reproduced and the density of the portion of that picture whichis or special interest, are effected by adjusting the condenser chargingcircuit to thereby vary the charge put on the timing condenser. a Themajor object of the present invention is to resistors to be wire woundvariable resistors.

The improvement gives important practical advantages. In particular, itpermits the use of individual resistor elements having resistance valuessmall enough so that each such element may be a wire wound resistor andhence have a stability and life length not obtainable when theresistances of the individual resistors are so high that it ispractically necessary that the latter be made of carbon. The inventionalso permits of a widerrange of variation in the condenser charge andhence in the exposure period than is practically possible with the priorarrangement. Furthermore, the improved arrangement permits compensatingadjustments of the condenser charges to be made in a suitablemultiplicity of predetermined steps. the relativeoperative efiects ofwhich are not significantly varied by the corrective adjustment madenecessary by variations in the emulsion characteristics of thephotographic material on which the pictures are printed. a

The various features of novelty which characterize my invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For a better understanding'of the invention,however, its advantages. and specific obiects attained with its use,reference should be had to the accompanying drawing and descriptivematter in which I have illustrated and described a prefererd embodimentof the invention. 7

The one figure of the drawing is a diagram illustrating a preferredembodiment of the present invention.

The apparatus diagrammatically illustrated in the drawings, comprisesmeans for automatically controlling the-duration or the exposure periods01' a projection printer A. The latter as diagrammatically illustrated,comprises a source oi printing light A' which ordinarily is a tungstenfilament lamp, a support A for transparency to be reproduced, anobjective lens A, a shutter A and a support A for photographic materialon which a transparent picture positioned by the support A may bereproduced by light transmitted through the picture and the lens fromthe lamp A to the photographic material on the support A when theshutter A is open. The shutter A is opened and closed by shutteroperating mechanism B when an energizing circuit for the latter isopened and closed-by the adjustment or a switch member C between openand closed positions.

The energizing circuit closed when the switch member is in its closedposition comprises a conductor i connecting a stationary switch contactto a supply conductor L, and a conductor 2 connecting a secondstationary switch contact to one terminal of the shutter operatingmechanism B. The second terminal of the device B is connected by theconductor 3 to a supply conductor L. The conductors L and L may supplyalternating current of ordinary voltage and frequency. For example,110-115 volts and 60 cycles per second. The stationary switch contactsto which th conductors I and 2 are connected, are bridged by the switchmember C when the latter is in its closed position. The switch member Cis biased to its closed position as by means or a spring 0' but isconnected to a relay armature D which cooperates with the relay windingE to hold the switch member 0 in its open position when said winding isenergized. The winding E is normally energized as hereinafter explained,but may be deenergized to initiate an exposure period by the momentaryopening of a normally closed switch F. A second switch member G shown asrigidly connected to the switch memher 0 and relay armature D controlsthe condenser charging circuit, as herein described. A; the switchmember 0 is adjusted between its closed and open positions, the switch Gis respectively adjusted between its open and closed positions andthereby opens and closes the condenser charging circuit.

As diagrammatically illustrated, a prism H passes some of the lighttransmitted through the picture on the support A to the cathode of aphotocell H and makes the latter conductive. When the deenergization oithe relay winding E causes the switch G to be moved into its openposition, the photocell H initiates a current flow through a condenserdischarging circuit and thereby effects the removal of all but apredetermined portion of the charge which the timing condenser I hadpreviously acquired in a manner hereinafter described. The condenserdischarging circuit comprises a conductor I connecting the anode of thephotocell H to one end of a resistor 8, and a resistor I which connectsthe second end oi. the resistor C to one end of a conductor 8. The otherend of the conductor 8 is connected to the terminal or plate i of thecondenser I. The second terminal or plate 1 of the condenser I, isconnected by a conductor 9 to the cathode oi the photocell H. Theresistors 8 and 1 form part oi' a voltage divider, the junction oi theconductor and resistor 8 being connected through a resistor HI andconductor II to the positive supply conductor DC or a direct currentdistribution system. The negative terminal DC or said distributionsystem is connected through conductors l2, l3 and i4 and a resistor IIto the Junction of the resistor l and conductor I. The voltage acrossthe reliltors l and 1 due to their connection to the D. 0. supplyconductors DC and DC, causes a current flow in the condenser dischargingcircuit when the photocell H is made conductive by the light passing toit and the switch G is open.

The direction 0! the condenser discharging current ilow is irom thecathode of the photocell H to the condenser plate or terminal 1. andfrom the second condenser terminal or plate 1 through the conductor 8,resistors I and 8 and conductor I in the order stated, to the anode oithe photocell. As hereinafter explained, the charge previously acquiredby the condenser I when the switch G is closed, makes the condenserplate or terminal i positive relative to the plate or terminal 1'. Thecondenser discharging current continues to flow until a predeterminedportion of the original condenser charge is eliminated. The portion orthe condenser charge thus eliminated is that required to make anelectronic valve J conductive. The valve J is a triode, shown as havingits control grid connected by a conductor It to the conductor a andhence to the negative terminal or plate i or the condenser I. Thecathode of the valve J is connected by a cathode bias resistor ii to theconductor 8 and thereby to the positive terminal or late 1 of thecondenser I. The anode of the valve J is connected to the supplyconductor DC by a resistor i1 and the conductor II. The charge acquiredby the condenser I prior to the opening or the switch (3 makes thecontrol grid 0! the valve J suillciently negative relative to thecathode to make the tube J non-conductive and to keep it non-conductiveuntil the end of the 40 exposure period. As the condenser charge isbeing reduced by the condenser discharging current flow, the potentialor the control grid 01' the valve J progressively increases relative tothe potential 01' the valve cathode, until the valve becomes conductive.

As shown, the upper or positive end oi. the resistor 'i is connectedto'ground by the ground connection It, and current flow through thecathode resistor II, which occurs when the valve J ismade conductive,increases the potential of the cathode oi the valve J relative to theground potential, and causes a previously non-conductive valve'K tobecome conductive, and thereby eil'ects the reenergizatlon oi the relaywinding E and the adjustment or the relay armature D to the right. Theresultant opening movement to the right 01 the switch C deenergizes theshutter mechanism B and thus terminates the exposure period initiated asa result of the deenergization of the relay winding B. The adjustment oithe switch member D to the right also moves the switch member G into itsclosed position and thus closes the hereinafter described condensercharging circuit. The valve K is a thyratron having its cathodeconnected to ground and having its control grid connectedby a conductorII to the cathode of the valve J. As shown, the thyratron K has a screengrid connected by a conductor 20 to a slider contact 2i engaging andadjustable along a potentiometer resistance 22 in shunt to a voltagecontroller shown as a glow tube L having its cathode connected to thesupply conductor DC by the conductor, and having its anode connected toa ground connection 23. In practice, the glow tube L may well'be of suchtype and form as to maintain a potential diflerence between its anodeand cathode of 150 volts, so that the potential or the negative supplyconductor DC is 150 volts negative relative to the ground potential. Thepotential oi the positive supply conductor DC may well be, and isassumed to be 250 volts positiverelative to the ground connection.

'The switch F may well be and is assumed to' be of a self-closing typeand is closed except when momentarily opened to initiate an exposureperiod. The opening of the switch F opencircuits and deenergizes thethyratron K as well' potential of the control grid of the thyratron Krelative to the potential of the thyratron cathode to fire the latterand thereby reenergize the relay winding B and almost instantly givesthe condenser a charge which makes the valve J not conductive. Theresultant decrease in the potential of the thyratron control gridrelative to the potential of the cathode of the thyratron does not makethe latter non-conductive, as-is well known.

The movement of the switch member G into.

its closed position, effected when the relay winding E is reenergized,closes a charging circuit for the condenser I. The condenser chargingcircuit, closed by the closure of the switch G, comprises one stationaryswitch terminal connected to the negative terminal or plate 1" of.

the condenser I, and a cooperating stationary switch contact connectedto a conductor 24 and connected to the first mentioned stationary switchcontact by the switch member G when the latter is in its closedposition. The charging circuit also includes the previously mentionedconductor 8 and resistor l5, and includes voltage divider elements M andN through which the conductor 24 is connected to the resistor l5.

The voltage divider element M comprises a plurality of resistancesections 25-34, connected end to end in series, a separate tap 35through which each of said resistance sections 25-34 is connected to acorresponding one of a plurality of separate switches 25'-34'. Each ofsaid switches 25'-34' when closed connects the corresponding resistancesection to the conductor.

24. In normal operation only one of the switches 25'-34 is closed at thesame time. The free end of the resistance section 25 is connected by aconductor 31 and slider contact 38 to-a slide wire resistance 39included in the circuit of the voltage divider N. The free end of theresistance section 34 of the voltage divider M is connected by aconductor 48 to a pivoted switch arm 4| adapted to engage one or anotheroi! a circular series of eleven switch; contacts 42. Each of thosecontacts is connected by a corresponding conductor 43 to one end of oneor another of ten series connected resistance sections 45-54. The end ofthe resistance section 45 not connected to the adjacent resistancesection 45, is connected to one end of the resistance 39.

v The other end of that resistance is connected to 6 the anode of theglow tube L and to the corresponding ground connection23. The end of theresistance section 54 not connected to the adjacent resistance section53 is connected through a resistor 55 to thejunction of the conductorsI4 and i3, and through the lager is connected to the cathode or negativeterminal of the glow tube'L'.

with the voltage divider elements M and N arranged as described, theresistance sections 38,

45-54 and the resistor 55 of the voltage divider N are connected inseries across the 810w tube L, and the resistance sections=25-34 of thevoltage divider element M are connected across the portion of theresistance of the voltage divider N, between the point along theresistor 39 engaged by the slider contact 38, and the point at the leftend of whichever ofthe resistance sections 45-54 is then connected bythe corresponding conductor 43 and switch contact 42 to the switch arm4i.

In the neutral or mid-position of the switch arm 4|, shown in thedrawing, it connects conductor 40 to the conductor 43 which is connectedto. the connected ends of the resistors 49 and 50. With that switch ar'mposition, the voltage divider M is connected in shunt to the portion orthe voltage divider N including the resistors 49, 48, 41, 45 and 45 andthe portion of the resistor 39 between the slider contact 38 and theresistor 45. As the switch arm 4| is rotated clockwise into engagementwith the lower right hand switch contact 42, the resistance sections 49,48, 41, 46 and 45 are successively eliminated from the portion of thevoltage divider N to which the voltage divider M is in shunt. Thisraises the potential of the negative plate 11' of the condenser I andthus decreases the charge put on the condenser I and consequently 40shortens the exposure period. When the switch mm 4| is turnedcounterclockwise from its midposition into engagement with the lowerleft hand switch contact 42, the resistors 58, 5|, 52, 53 and 54 aresuccessively added to the portion of the voltage divider N to which thevoltage divider M is in shunt and the exposure period is correspondinglylengthened.

The operative effect of the voltage divider N on the charge given thecondenser I preparatory to an exposure, depends upon which of theswitches 25-34 is closed. In the neutral adjustment of the voltagedivider M, the switch 30' is closed and the remaining switches of thevoltage divider are open. With the switch 38' closed, the potentialgiven the negative condenser plate 1'. is the ground potential less thevoltage drop in the portion of the resistance 39 between the groundconnection 23 and the slider contact 33, plus the voltage drop in theresistor sections 25, 25, 21, 28 and 29, and the portion of theresistance section 30 at the right of its connection to thecorresponding tap 35. The actual potential drop occurring in each of theresistor sections 25 to 34 depends upon the position oi. the contact 38along the slide wire resistance 39 and on the adjustment of the switcharm 4|, and does not depend on which of the switches 25-34 is closedsince there is no current flow through the closed switch, except closingthe switches 292128, 2?, 26' and 28,

one at a time, is to successively increase the po tential of thecondenser plate 2, and to corr spcnolingly decrease the exposure period.Similarly, the effect of successivel closing the switches 32', and oneat a time, is to successively decrease the potential of the condenserplate 2" and to correspondingly increase the exposure period. I

In the practical use of the invention, the switches '3 3 are ordinarilyadjusted as re= quired to compensate for differences between the averagedensity of each transparent picture re-= produced anol the density of aparticular interest portion of the picture, when the difference is of acharacter to require compensation. need for compensation can bedetermined with substantial accuracy by a suitably skilled. operatorfrom a visual examination of the picture. The need for such compensationcan he deter= mined more easily and more accurately by the use of anautomatic coder which provides a measure of the ratio of the density ofthe special interest portion of the picture to the average density. ofthe picture, as is explained in the above-mentioned Eicctronics article.In the practical use of the present invention, in the commercialreproduction of transparent pictures, each picture is coded and thenpassed to the oper= ator of the projection printer by which the pictureis to be reproduced, with a code indication as to which of theparticular switches re -3c should be closed in reproducing the picture.

The accuracy with which the needed compensation results are obtained incommercial operation, is increased by suitably relating the values ofthe resistance sections between each adjacent pair of taps so. Inpractice, the resist ance sections are advantageously arranged so thatthe efiect of opening one of the switches 225 3 previously closed, andthe closure of the adjacent switch at the left of the switch opened, isto increase the exposure period 1.41 1 /2) times. This means that theclosure of any one of the switches following the opening of the secondswitch to the right of the switch closed doubles the exposure period forthe same transparency. Similarly, the eflect or closin any one of theswitches following the opening of the second switch to the left of theswitch closed, is to shorton the exposure period by one-half.

It is not essential, of course, that there should be any particularnumber of switches associated with the resistance sections of thevoltage divider M, or that the resistance should have their resistancevalues related as just described. Regarolless of the number of switchesand the relative values of the associated resistance elements of thevoltage divider M, it is practically desirable, however, that thevoltage dividers M and N be associated with one another in the condensercharging network in the general manner de= scribed. When the voltagedividers are so associated, the adjustments of the switch arm M neededto compensate for variations in the sen sitivity or speed of thephotographic printing material coated with emulsions made in differentbatches, do not change the relative efiects of the much more frequentlymade adjustments. of the different switches 25'-34' of the voltagedivider element M needed to compensate for the difference between theexposure time actually needed in reproducing a particular transparentpicture, and the exposure time for the picture The all

correspondin to and based, directly on the aver age density of saicipicture.

In ordinary practice, the various switches may advantageously beactuated by an electromagnetic relay mechanism including selector switchpush buttons conveniently located for actuation by the operator, in thegeneral manner disclosed in the prior application of Jacob Rabinowitz,Ser. No. 3594, 163, filed November it,

192 5. In such case, it is ordinarily convenient to I have the switchactuating relay system arranged so that at the conclusion of theprinting of one or more pictures from any particular transparency, theswitch system is left in, or is automati cally adjusted into thecondition shown in the drawing in. which the neutral or mioi=positionswitch so is closed and all of the other switches ES 6' are openeol.

For the convenience of the operator of the printing machine the switchesEl -343', or the push buttons through which they are controlled, areconveniently designated by suitable symbols, such as the symbols V, -IV,lll, -11, I, it, +1, +31, +Illl and +IV shown in the drawing asrespectively associated in the order stated with the switches 2E, 2?",etc; 8G respectively. When the resistors 25-36 are wire wound resis torsof commercial magnitudes, the tap connec tion 35 to each resistor may heaohusteol along the length of the latter in assembling the unit M so asto properly relate the resistance values oi the sections of the voltagedivider hetwecn'thc different switches 25, 26, etc.

In an embodiment of the invention now in commercial use, the switch armM is adjusted from time to time as required to correct forvaria tions inthe sensitivity of the emulsion coating of the photographic material onwhich pictures are being printed. The quantity or batch or photo-vgraphic material coated with the same emulsion ordinarily put in stockin a commercial printing plant may well be so large that thousands ofpictures are printed from said batch. In said commercial embodiments ofthe invention, the various resistors of the voltage divider N have suchresistance values that an adjustment of the switch arm in the clockwisedirection from any one contact to the immediately adjacent contact, willincrease the exposure period approximately 10%,

and each similar adjustment step or the switch arm 48 will shorten theexposure period approximately 10%. In said embodiment, the resistancevalues of the various resistors in the voltage dividers M and N areapproximately as follows:

Resistance es s l s ys ewws $3 5sss swl By way or further illustrationand example, it

is noted that the resistors I and I! may have resistance of 1000 and25,000 ohms, respectively, and that suitable resistance values for theresistors 5, I0, I! and I1 are 15,000 ohms, 25,000 ohms, /2 megohm and 1megohm. It is to be noted, however, that while the resistance valueshereinbefore stated are suitable, and suitably related, for practicaluse, none of those resistance values is sharply critical. The capacityof the timing condenser I should be suitablyrelated to the conditions ofuse, and in particular to the magnitude oi the charging voltagesimpressed on it, and to the voltage difference between the condenserplates 2 and i at the instant in which the tube J becomes conductive andoperates through the thyratron K and relay winding E to close the switchG and thus interrupt the condenser discharging operation. In practice,with the resistance values above stated and with 150 volts across theglow tube L, the capacity of the condenser I may vary between .05 and.006 microfarad, and the maximum voltage impressed on the condenserplates 1 and i may vary from a maximum of .140 volts to a minimum ofabout 5 or 6 volts. With a condenser having a capacity of .02 microiaradand with an effective charging voltage of 140\volts, the exposure periodused in reproducing a transparent picture 01 average density on photograhic printing material of average sensitivity, will be about 30 seconds.With the same condenser, transparent picture and printing material, theexposure period may be about one-half of a second when the effectivecharging voltage is It is believed that the operation of the apparatusillustrated and described herein has now been made appar t.

The timing mechanism illustrated and described herein comprisesimprovements on the invention disclosed and claimed in theabovementioned prior application of Jacob Rabinowitz.

While in accordance with the provisions of the statutes, I haveillustrated and described the best form 0! embodiment of my inventionnow known to me, it will be apparent to those skilled in the art thatchanges may be made in the form 01 the apparatus disclosed withoutdeparting from the spirit of my invention, as set forth in the appendedclaims, and that in some cases certain features of my invention may beused to advantage without a corresponding use of other features.

Having now described my invention, what I claim as new and desire tosecure by Letters Patent is:

1. In a projection printer comprising,mechanism for reproducing atransparent picture by projecting light through the picture tophotographic printing material and comprising exposure timing meansincluding a timing condenser having two terminals and means forproportioning each exposure period to the time required to effect apredetermined reduction in the charge previously given said timingcondenser by a discharge current which flows through a photocellincluded in said mechanism and rendered conductive by light projectedthrough said picture, and which is proportional to the average densityof said picture, the combination with said mechanism, of means forvarying th charge put on the condenser preparatory to an exposurecomprising a voltage source, a voltage divider having resistancesections connected in series across said source, a second voltagedivider having resistance sections connected in series with one another,switching means operative to connect said second voltage divider inshunt to a variable portion of the first mentioned voltage divider, andmeans for connecting one terminal of said timing condenser to saidsecond voltage divider at one or another or a plurality of of pointsdiffering in potential from one another, and a source of predeterminedpotential to which the second terminal'of said condenser is connected.

2. A combination as specified in claim 1, in which said switching meanscomprises a plurality of contacts, a switch arm connected to one end ofsaid second voltage divider and adjustable into diiferent positions tothereby engage the different contacts one at a time, and conductor meansconnecting the different contacts to said first mentioned voltagedivider at respectively different points along the length of the latter.

' WALTER L. LINDE.

No references cited.

